The present invention relates to a hydraulic brake system for a land vehicle having a master cylinder, which is actuable by the driver by means of a pedal directly or via a brake booster disposed downstream of the pedal, for generating brake pressure in two brake circuits, each of which comprises two wheel brakes. In this case, a hydraulic connection between the master cylinder and the wheel brakes is to be established or blocked by means of valve arrangements in dependence upon trigger signals from an electronic control unit in order that normal braking operations brought about directly by the driver or braking operations influenced by the electronic control unit may be carried out in dependence upon measured quantities, which are supplied to the electronic control unit and reflect the vehicle behaviour and/or the driver request, by varying the brake pressure characteristic in the wheel brakes by activating the valve arrangements associated therewith.
Such hydraulic brake systems are able and intended to carry out automatic braking operations. By these are meant braking operations that occur as a rule independently of a force introduced by the driver at the pedal. Examples of these are braking operations for acceleration spin regulation (ASR), which prevents individual wheels from spinning during the starting operation by targeted braking of the corresponding wheels, braking operations for an electronic stability program (ESP), which adapts the vehicle behaviour in extreme ranges to the driver request and the road conditions by targeted braking of individual wheels, or braking operations for adaptive cruise control (ACC), in which i.a. by automatic braking a defined distance of the actual vehicle from a vehicle travelling in front is maintained.
Such automatic braking operations, which, caused by corresponding input variables (wheel speeds and/or vehicle speed, or the like) have to run off at low brake pressure level, are particularly susceptible to a jerking and/or pulling of the vehicle that is perceptible by the driver. This is the case for example during ACC braking interventions. These situations make it necessary for almost the same brake pressure levels to prevail in both brake circuits I, II during the braking operation in order to prevent such a jerking and/or pulling of the vehicle from occurring.
The underlying problem of the invention is that during automatic braking operations the two brake circuits are hydraulically uncoupled from one another, with the result that a pressure equalization between them cannot occur.